Re: thread-ready ABIs

["Tommy S. Christensen" <tommy.christensen@eicon.com>]

Dominic Sweetman wrote:
> 
> > In any case, that's not the real problem.  Linux user threads do not
> > have true separate stacks.  They share their _entire_ address space;
> > the stacks are all bounded (default is 2MB) and grouped together at
> > the top of the available memory region.
> 
> Quite.
> 
> A comment by Kevin reminded me of the real constraint (which the
> experts probably take for granted): this system is supposed to work on
> shared-memory multiprocessors and multithreaded CPUs.
> 
> In both cases two or more threads within an address space can be
> active simultaneously.  On a multithreaded CPU (in particular) there's
> only one TLB, so memory (including any memory specially handled by the
> kernel) is all held in common.  The *only* thing available to a user
> privilege program which distinguishes the threads is the CPU register
> set.
> 
> (Well, and the stack, which is a difference inherited from the value
> in the stack pointer register.  But the stack pointer is not really
> going to help much to return a thread-characteristic pointer or ID.)

Well, why not use the stack?

I am not quite familiar with the requirements on this "thread register",
but couldn't something like this be made to work:
  #define TID *((sp & ~(STACK_SIZE-1)) + STACK_SIZE - TID_OFFSET)

It assumes a fixed maximum stack size (and alignment), which it should
be possible to meet (virtual memory is cheap). The STACK_SIZE could
probably even be a (process global!) variable if it is not desirable
to limit this at compile time.

  -Tommy